Accurate depth measurement is an important parameter when performing a logging operation in an oil well. Inaccuracies in these measurements can cause tremendous problems in reservoir evaluation, in reservoir management, and in calculating reserves, among other problems. For wireline logging operations, a cable spooling and measuring device may be used to measure the spooled cable length. This device includes a pair of measurement wheels, through which a cable is spooled. These wheels are pressed from opposite directions to the cable and rotate in unison as the cable moves therebetween. With this arrangement, the length of the cable passing through the wheels can be measured by measuring the rotation of the wheels and knowing the circumference of the wheels.
However, this system has inherent shortcomings. For example, the quality of the measurement relies largely on the assumption that there is no slippage between the cable motion and the wheel rotation. Yet, this assumption is not always valid, especially in situations where the cable speed is high or when the cable abruptly changes directions of motion.
In addition, the wheels themselves are subject to wear and tear, which over time causes a groove in the wheels, which changes the diameter of the wheels and causes for an inaccurate measurement of the cable depth in the well. Also, the wheels are subject to damage by corrosive mud and debris on the cable, which can also change the diameter of the wheels. As such, the device must be recalibrated on-site (in the field) in order to account for wear and/or other damage to the measurement wheels. Also, heavily worn/damaged wheels must be replaced entirely.
Accordingly, a need exists for an improved system and method for measuring the movement of a conveyance system relative to an oil well during an oil well operation.